A. Field of the Invention
The present invention relates generally to the recovery and regeneration of used lubricant and industrial oils. More specifically, the present invention relates to the treatment and refinement of used lubricants and industrial oils to produce re-refined base oil and to remove additives and impurities from used oils and lubricants.
B. Description of Related Art
Commercial, residential and industrial machinery generate large amounts of used oils. In particular, the maintenance of mechanical devices with active components generates a considerable amount of used oil. In this context, used oil refers to used lubricants and industrial oils, whether they are used for lubricating, hydraulic, or other purposes, in mechanical, electrical electrical or other types of equipment. Used oil also includes synthetic oils and mineral oils that are obtained from natural sources.
Used oil does not retain the characteristics that made the original base oil suitable for its intended purpose, and it must be disposed of or re-refined. Burning used oil is one way of disposing of it. Although this causes significant pollution and releases many hazardous metals into the air, burning of used oil is accepted as a used oil disposal method. Whereas burning used oil dramatically reduces the amount of solid and liquid material that must eventually be disposed of, the volatile products that are emitted during incineration are found to be unacceptable under some environmental risk assessment and management schemes. In addition, incineration of used oil leads to calcination products and dissipated thermal energy. Incineration products may give rise to environmental pollutants, depending on the substances that are present in the incinerated products. Thermal energy that is dissipated in the environment may be an unacceptable source of thermal pollution. Finally, incineration of used oil products may also cause environmental pollution through the emission of gases and particulate matter that increase air pollution or contribute to detrimental greenhouse effects. Due to the problems associated with the burning of used oil, reprocessing (or re-refining) is an alternative that should be considered on environmental and economic grounds. In addition, some countries have banned the burning of used oil.
Used oil can be reprocessed into useful base oil. To be economically efficient, the oil must be regenerated by a process that preserves the useful characteristics of the oil, but removes additives and impurities. This recovery and regeneration goal faces the challenge imposed by the complex chemical composition of used oil. For most applications, oil is a mixture of a base oil and a number of additives; the additives enhance the useful characteristics of oil. For example, engine oil typically contains rust inhibitors, antioxidants, antiwear agents, detergent-dispersants, antifoaming agents and viscosity index improvers. When oil is used, however, the constituents of the base oil and the additives break down through a series of chemical and thermal reactions. These reactions create a complex mixture of chemical species.
For example, in addition to degraded and broken down additives, used engine oil contains metals and metal compounds that include lead, iron, calcium, zinc, sodium and magnesium, and phosphorous, sulfur and nitrogen containing compounds. This complexity is the main challenge faced by any used oil re-re-refining method. For example, some of the chemical species present in used oil directly interfere with re-refining operations. Other chemical species react during re-refining and form products that interfere with subsequent re-refining steps. Finally, some chemical species are not removed by the re-refining process and interfere with intended uses of the re-refined base oil. As used herein, re-refining and its related terms refer to the series of recovery and regeneration operations that transform used oil into a useful base oil by removing additives, asphaltic material, and other impurities. A base oil is a re-refined oil that can be used in industrial applications in general and as a lubricant in particular. Base oil that is substantially free of impurities can also be used as a feed stock to petroleum refinery process units including the fluid catalytic cracking unit. Additives can be incorporated into the re-refined base oil when the specific application so requires. Asphaltic material and impurities consist of a heavy fraction, or high molar mass fraction, and breakdown products of the oil and its additives. The asphaltic material and other impurities can form a residuum (often simply called resid) which is useful as a asphalt extender.
The impurities that must be removed from used oil during re-refining include acids, water, suspended solids, light hydrocarbons, glycols, sulfur compounds, metals and organometallic compounds, and mixtures of a variety of polymers. In particular, the removal of organometallic compounds and polymers from used oil is difficult because they are hydrophillic, and most of them vaporize at a temperature that is very similar to that at which the base oil vaporizes. In addition, used oil has a high viscosity which is detrimental to the distillation step in the re-refining process.
Polymers and metals, including zinc, calcium, and phosphorous compounds, are among the impurities that must be eliminated during re-refining to obtain a good quality base oil and to avoid fouling of re-refining equipment. In particular, the elimination or reduction of zinc compounds in used oil to a non-interfering level is very important for avoiding downstream re-refining equipment fouling. Zinc is an active species in polymerization reactions and it is present as a zinc dialkyldithiophosphate (ZdP), among other forms. ZdPs and related compounds are added to oils because they act as antioxidant agents. They also reduce the oil's wear as a lubricant, and they protect metals against corrosion. Nevertheless, ZdPs undergo hydrolysis and thermal decomposition. ZdP decomposition has been studied by, for example, J. J. Dickert and C. N. Rowe, Journal of Organic Chemistry, Vol. U32 (1967), pp. 647 et seq. This reference is incorporated by reference herein. ZdP is a source of acidic compounds, and it eventually forms insoluble zinc phosphate that causes fouling of re-refining equipment. Thermal decomposition of ZdP usually produces a glassy insoluble polymeric solid that is one of the major causes of re-refining equipment fouling. This fouling includes the plating out of the polymers in heat exchangers and distillation columns.
One method of processing and re-refining used oil uses propane as a solvent to extract oil. For example, U.S. Pat. No. 2,070,626 describes a process for mixing used oil with liquid propane to remove asphaltic materials and other oxidation products. This patent is incorporated by reference herein. Similarly, U.S. Pat. No. 2,196,989 describes a process for separating asphaltic compounds from crude oil to produce a lubricating oil. This patent is incorporated by reference herein. In this process, the oil is mixed with a light hydrocarbon solvent such as liquid propane or butane. A substantially inert gas precipitates impurities from the oil-propane mixture. U.S. Pat. No. 3,870,625 describes another process for de-asphalting residues from the vacuum distillation of petroleum or from used mineral oils. This patent is incorporated by reference herein. The oil or distillation residue is injected under pressure into liquid propane in a pulsed manner to facilitate dispersion of the oil material in the solvent as fine droplets. Improvements to this process are disclosed in U.S. Pat. No. 4,265,734, which is also incorporated by reference herein. Finally, U.S. Pat. Nos. 5,286,380 and 5,556,548 disclose an apparatus and method for removing contaminants from used motor oil by means of extraction with a liquid hydrocarbon and a gas. These patents are incorporated by reference herein.
A major disadvantage of these prior methods is that they do not remove certain contaminants from used oil. For example, because ZdP and certain zinc-based impurities are soluble in organic phases, extraction with propane does not efficiently separate them from the oil. Furthermore, the presence of metal and polymeric compounds causes fouling of the re-refining equipment when they are present in the used oil.
U.S. Pat. No. 4,376,040 discloses a process for treating and regenerating used oil products that relies on the agitation of used oil and a quaternary ammonium salt or mixture of salts. This patent is incorporated by reference herein. The claimed process relies on the use of a quaternary ammonium salt that contains at least one aryl ring, or a mixture of such salts. Reportedly, this process does not remove or deteriorate oil additives.
A pretreatment process that uses a basic compound of an alkali metal for avoiding acidification and fouling in the re-refining of waste lubricating oil is disclosed in Canadian Patent No. 2,068,905. This patent is incorporated by reference herein. This pretreatment process does not utilize a phase transfer catalyst for assisting in the chemical reactions that cause the break down of impurities into compounds that will separate in a subsequent liquid-to-liquid extraction.
PCT publication no. WO 98/26031 (based on PCT patent application no. PCT/FR/02224) discloses a process for re-refining waste oil. That publication is hereby incorporated by reference.
Another method for reprocessing used engine oil is a vacuum distillation process followed by hydrotreating. In this process, waste oil is heated to about 150.degree. C. to remove any water as well as light hydrocarbons. The dewatered oil, containing additives, is then heated to about 260.degree. C. to remove any diesel fraction. The oil with its additives is then heated to about 370.degree. C. in a distillation column operating at about 5 mm Hg absolute to separate the base oil from the additives and the base oil distillate is then hydrotreated to improve color and odor. Hydrotreating also removes a portion of residual polycyclic aromatic compounds. The main problem with this process is that the additives and their breakdown products are not removed until the distillation step. By then the oil, additives, and additive derivatives have been heated to over 370.degree. C., at which temperature thermal cracking of the polymeric compounds and thermal decomposition of the organo-metallic compounds produce severe coking and corrosion in the distillation column and ancillary plant. Coking and corrosion of the plant not only interferes with throughput efficiencies but also results in poor quality lube oil distillate.
Generally speaking, thin film evaporators of the type employed in the process described above are expensive to construct and operate on a per unit throughput capacity. In addition, about 2% of the light oil (diesel) fraction is lost in this process in the water removal stage and about 3% of the available base oil is lost in the final distillation stage due to retention in the asphaltic component.
Several lubricating oil compositions that incorporate quaternary ammonium salts are known. These include the compositions disclosed in U.S. Pat. Nos. 4,388,200, 4,273,663, 4,253,980, 4,251,380, 3,962,104, and 5,126,397. A quaternary ammonium salt has been employed in a process for removing catalyst residues from olefin polymerization products, as disclosed in U.S. Pat. No. 5,196,630. Each of the aforementioned patents and references is hereby incorporated by reference in its entirety for the material disclosed therein.
U.S. Pat. Nos. 4,624,763, 4,624,764, 4,661,226, 4,634,510, 4,627,901, 4,622,119, and 4,622,118 describe the removal of waxes from lubricating oil by the induction of a high voltage charge into oil-solvent mixtures to obtain nucleation of wax particles before precipitation. These patents are incorporated by reference in their entirety for the material disclosed therein.
Australian Patent Number 605,288 and U.S. Pat. No. 5,066,386 describe processes for extraction of oil from stable oil-water emulsions. This patent is incorporated by reference herein. In this process, a liquefied hydrocarbon solvent is added to the emulsion forming an oil solvent phase. The pressure is then reduced to allow the pressurized liquid solvent to vaporize, whereupon the emulsion breaks into oil and water phases.
In addition to removing metals, organometallic compounds and polymers from oil, re-refining methods also need to efficiently remove oil additives and the breakdown products of the additives. For many uses of re-refined base oil, the presence of additives and breakdown products in the re-refined oil is unacceptable. For example, viscosity index improvers are added to oil to widen the temperature range within which the oil retains a certain viscosity. These additives are usually polymers at a concentration that can be as high as 12% by weight. Viscosity index improvers, however, can undergo a variety of thermal and oxidative reactions while the oil is used and in re-refining operations. Thermal processes may lead to depolymerization and pyrolysis of ester chains to form olefins and acids. Oxygen and in general any source of free radicals may oxidize viscosity index improvers. Once a polymer radical is formed, polymer backbone cleavage is possible. In essence, most of these additives are significantly degraded at temperatures of 260.degree. C. or above. Thermal or oxidative degradation, or a combination of both, will lead to used oil with a complex mixture of additives and additive degradation products. Treatments that do not significantly alter the additive composition of a used oil may not produce a base oil that is free from additives or degradation products. Testing has shown that extraction with propane alone does not completely remove viscosity index improvers from used oil. Furthermore, subsequent re-refining during oil re-refining subjects oil to temperatures above 260.degree. C., which causes remaining viscosity index improvers to decompose during distillation. These decomposition products will cause fouling of the re-refining equipment. Thus, there is also a need for an oil re-refining method which efficiently extracts additives and breakdown products in general, and viscosity index improvers in particular, from used oil.
It is notable that the most successful prior art methods for recovering base oil from waste oil include placing a caustic in the waste oil and heating the waste oil to a very high temperature, such as 650 degrees Fahrenheit, thereby causing hydrocarbon cracking and distillation in order to purify the waste oil. As will be seen below, the invented method does not involve the use of high levels of heat or the use of hydrocarbon cracking in the pretreatment process where most contaminants are removed from the waste oil.
The limited scope of current used oil re-refining methods leads to fouling problems when a variety of used oils are treated and re-refined. Among the many prior art methods, there still remains a need for a process for pretreating and a process for re-refining used oil in order to remove impurities and generate a substantially clean base oil on an economically efficient basis. The invention satisfies this need.